Monitoring the water content in hydraulic brake fluids is known to be of crucial importance in maintaining efficient and effective braking function. During the braking, especially the sustained braking, friction-generated heat can cause a brake fluid to boil and form vapor bubbles that can cause a complete loss of the braking function. Since most of the commercially used brake fluids are highly hygroscopic, the water content in such a fluid can increase due to water absorption from the surrounding environment. If the water content in a brake fluid increases to 2%, the boiling point of the fluid decreases significantly, thus, decreasing the efficiency of the brakes and overall safety of the operating device.
Another area of concern related to the high water content in brake fluids is the increase of the fluid's corrosive activity that can cause corrosion of the braking system components that come in contact with the brake fluid. Antilock braking systems (ABS) that are presently installed on many motor vehicles are especially susceptible to corrosion. Having water in a brake fluid during the period of about 2 years irreparably damages an ABS unit. A replacement for an ABS unit presently costs about $1,800.
Two methods of measuring water content in a brake fluid are employed in the products currently available on the market. The Karl Fisher titration method is used in a complicated device that costs around $7,000. Another type of a device that uses the method of measuring the boiling point of a brake fluid costs about $1,000. It is obvious that the majority of service stations will not be able to afford and use expensive measuring devices. Yet another method of determining water content in brake fluids by measuring conductivity of a fluid has proven to be inaccurate. Therefore, it is highly desirable to have a quick, easy and inexpensive method and device for determining water content in a brake fluid. Such method and device would allow service technicians to measure water contents in brake fluids and recommend brake fluid changes before either safety or corrosion concerns arise.
Refractometers have long been used as field instruments for a variety of applications. They have been useful in determining sugar content in grape juices, water content in honey, the concentration of water soluble metalworking lubricants and protein levels in blood serum. American Society of Testing and Materials Standard Practice D 3321 details use of a refractometer for determining the freeze point of engine coolants with far greater accuracy than traditional hydrometer methods. Refractometers require a very small sample size. Refractive index measurements are quick and can be performed with simple, relatively inexpensive instruments. Therefore, refractometry can be used to determine a percentage of water in fluids by measuring the critical angle of total reflection in a particular fluid while providing for temperature compensation that becomes necessary due to the temperature dependence of the refractive indices. Since the refractive index n of a fluid changes as the water content increases, fluids with different water contents will have different refractive indices, and, therefore, different critical angles of total reflection. Due to the phenomenon of total reflection there will be a boundary between the dark and light portions of a scale in the observation area. The location of the boundary on the scale than is calibrated in water content units will indicate the percentage of water in a tested sample of a fluid.
Refractometers that use the critical angle of total reflection to determine percentages of water in such substances as antifreeze coolants have been known in the industry. For example, U.S. Pat. No. 5,355,211 to Thompson et al. (assigned to Leica, Inc.) discloses a refractometer subassembly method and apparatus comprising critical angle prism 214 and compensating wedge 232 disposed within housing 210, as shown in FIG. of that patent. Wedge 232 is attached to one end of temperature responsive member 234.
U.S. Pat. No. 4,243,321 "Handy Refractometer" to Okuda et al. discloses a handheld refractometer, as particularly illustrated in FIGS. 1 and 5 of that patent. The patent discloses mirror 12, one end of which is secured to holding member 14 for prism 3 by plate spring 13. The other end of mirror 12 is urged against thermally expansible member 15 by the force of spring 13. As temperature changes, the thermally expansible member expands or contracts, thereby rotating the mirror 12. In the Okuda patent rotation of mirror 12 about the pivot P keeps beam 53 parallel to original beam 52 in order to compensate for a temperature change.
U.S. Pat. No. 4,451,147 "Refractometer" to Dobes et al. discloses a refractometer that utilizes light refracting wedge 40, mirror 18, mirror surface areas 32 and 44, and prism 22, as illustrated in FIGS. 1, 2 and 3 of that patent. According to the Dobes patent, mirror 18 reflects beam 54 and directs it to prism 22. At the lower side of prism 22 the beam splits into reference light beam 56 and measuring light beam 58. Reference light beam 56 is then reflected by mirror surface 32 of prismatic body 42 and by partial mirror surface area 44 of light refracting wedge 40. Measuring light beam 58 gets refracted at active surface area 34, and passes through light refracting glass wedge 40 to be reflected by a mirror surface at the lower side 60 of wedge 40. Temperature compensation in that patent is achieved by directing both reference and measuring beams through the same prism, thus, exposing the two beams to the same temperature differences. For the Dobes invention to operate prism 22 has to be partially coated with a reflective coating.
Due to the nature of certain chemicals contained in brake fluids, there are two problems that are specific to brake fluids and that render the known refractometers completely inapplicable to measuring refractive properties of such fluids.
First, the range of the refractive index n in a brake fluid spans from about 1.4447 at 0% of water to about 1.4394 at 7% of water. This range is much narrower than that of a coolant (spanning from about 1.33 to about 1.38 over the same range of the percentages of water). Thus, a much more sensitive device reflecting a different refractive index range is necessary to design a brake fluid refractometer. The increased sensitivity requirement can be satisfied by achieving more accurate temperature compensation. The known method of temperature compensation employed in a refractometer for coolants uses a bimetallic strip attached to a temperature compensating prism (called "a wedge" in the following description of the present invention) and to the prism on which a sample fluid is deposited. The material of the compensating prism is such that its temperature coefficient (dn/dT) is negative if the temperature coefficient of the sample fluid is positive, and vice versa. The bimetallic strip deflects from its initial position when temperature changes, changing the angle of the attached compensating prism and, thus, providing for the correct temperature compensation in measuring the refractive index of a sample fluid.
The concept of using two materials with opposite temperature coefficients and a bimetallic strip has been known and used in the industry. It is described, for example, in U.S. Pat. No. 3,329,060 to Holleran and U.S. Pat. No. 3,267,795 to Goldberg which are incorporated herein by reference.
Such a design for temperature compensation proved to be unworkable in the case of measuring water contents in brake fluids. The temperature coefficient of a brake fluid is approximately 3 times smaller than the temperature coefficient of a coolant (-38.times.(10).sup.-5 /.degree. C. for a brake fluid, -12.times.(10).sup.-5 /.degree. C. for a coolant). Therefore, a much more sensitive temperature compensation device is needed to measure water content in a brake fluid over a temperature range typical for a regular service station. Because of the increased sensitivity requirements a bimetallic strip used in the refractometer for coolants can not provide the necessary angular compensation. Therefore, a new temperature compensation device is needed for a brake fluid refractometer.
A second problem arises from the fact that in a brake fluid the refractive index changes negatively with the increase of the water content, whereas, for example, in a coolant the refractive index increases with the increase of glycol. Thus, there exists a need of inverting the image on the reading scale so that the bottom of the scale will correspond to 0% of water and the top of the scale will correspond to 7% of water in a brake fluid. Such inversion is highly desirable because the bottom-to-top calibration is familiar to the customers and has been used in already existing coolant refractometers.
It is also greatly desirable to have a refractometric apparatus that would allow a user to determine not only the percentage of water in a particular hydraulic fluid, but also the boiling point of such a fluid. Such an apparatus would be especially useful, for example, in servicing race cars, because for servicing such cars properly it is the boiling point that needs be determined in order to prevent the loss of the braking function.